Autonomic nervous system

The autonomic nervous system (ANS) is that part of the peripheral nervous system that largely acts independent of conscious control (involuntarily) and consists of nerves in cardiac muscle, smooth muscle, and exocrine and endocrine glands. It is responsible for maintenance functions (metabolism, cardiovascular activity, temperature regulation, digestion) that have a reputation for being outside of conscious control. The other main subdivision of the peripheral nervous system, the somatic nervous system, consists of cranial and spinal nerves that innervate skeletal muscle tissue and are more under voluntary control (Anissimov 2006; Towle 1989).

The autonomic nervous system is typically divided into two main subsystems, the sympathetic nervous system and the parasympathetic nervous system. These tend to balance each other, offering opposite and yet complementary effects reflective of the philosophy of Yin and Yang. The sympathetic nervous system deals with the response to stress and danger, releasing epinephrines (adrenaline), and in general increasing activity and metabolic rate. The parasympathetic nervous system counters this, and is central during rest, sleeping, and digesting food and, in general, lowers metabolic rate, slows activity, and restores blood pressure and resting heartbeat, and so forth (Chamberlain and Narins 2005). Just as Yin and Yang are opposing, yet complementary and interdependent forces, the sympathetic and parasympathetic systems complement each other and are both necessary to create overall harmony and balance in the living organism.

The peripheral nervous system, in turn, is commonly divided into two subsystems, the somatic nervous system and the autonomic nervous system.

The somatic nervous system or sensory-somatic nervous system involves nerves just under the skin and serves as the sensory connection between the outside environment and the CNS. These nerves are under conscious control, but most have an automatic component, as is seen in the fact that they function even in the case of a coma (Anissimov 2007). In humans, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Chamberlin and Narins 2005).

The autonomic nervous system is typically presented as that portion of the peripheral nervous system that is independent of conscious control, acting involuntarily and subconsciously (reflexively), and innervating heart muscle, endocrine glands, exocrine glands, and smooth muscle (Chamberlin and Narins 2005). In contrast, the somatic nervous system innervates skeletal muscle tissue, rather than smooth, cardiac, or glandular tissue.

The autonomic nervous system is subdivided into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. In general, the sympathetic nervous system increases activity and metabolic rate (the "fight or flight response"), while the parasympathetic slows activity and metabolic rate, returning the body to normal levels of function (the "rest and digest state") after heightened activity from sympathetic stimulation (Chamberlin and Narins 2005). The enteric nervous system innervates areas around the intestines, pancreas, and gall bladder, dealing with digestion, and so forth.

Unlike the somatic nervous system, which always excites muscle tissue, the autonomic nervous system can either excite or inhibit innervated tissue (Chamberlin and Narins 2005). Most associated tissues and organs have nerves of both the sympathetic and the parasympathetic nervous systems. The two systems can stimulate the target organs and tissues in opposite ways, such as sympathetic stimulation to increase heart rate and parasympathetic to decrease heart rate, or the sympathetic stimulation resulting in pupil dilation, and the parasympathetic in pupil constriction or narrowing (Chamberlin and Narins 2005). Or, they can both stimulate activity in concert, but in different ways, such as both increasing saliva production by salivary glands, but with sympathetic stimulation yielding viscous or thick saliva and parasympathetic yielding watery saliva.

In general, the autonomic nervous system controls homeostasis, that is the constancy of the content of tissues in gases, ions, and nutrients. It does so mostly by controlling cardiovascular, digestive, and respiratory functions, but also salivation, perspiration, diameter of the pupils, micturition (the discharge of urine), and erection. While many of the activities of the ANS are involuntary, breathing, for example, can be in part consciously controlled. Indeed, although breathing is a purely homeostatic function in aquatic vertebrates, in land vertebrates it accomplishes much more than oxygenating the blood: It is essential to sniff prey or a flower, to blow out a candle, to talk or sing. This example, among others, illustrates that the so-called “autonomic nervous system” is not truly autonomous. It is anatomically and functionally linked to the rest of the nervous system and a strict delineation is impossible.

The ANS is nevertheless a classical term, still widely used throughout the scientific and medical community. Its most useful definition could be: The sensory and motor neurons that innervate the viscera. These neurons form reflex arcs that pass through the lower brainstem or medulla oblongata. This explains that when the central nervous system (CNS) is damaged experimentally or by accident above that level, a vegetative life is still possible, whereby cardiovascular, digestive, and respiratory functions are adequately regulated.

Sensory and motor neurons

Neurons active in the autonomic nervous system (and the PNS in general) can be divided into sensory neurons and motor neuron (Chamberlin and Narins 2005). Sensory neurons act as a conduit between sensory receptors, which sense stimuli such as cold, heat, and pain, and the CNS. Motor neurons act as a conduit between the CNS and various muscles and glands (effectors). Or, looked at another way, receptors are cells or groups of cells that receive information from stimuli (external or internal), and effectors are cells or groups of cells that received information from the nervous system.

Although commonly the ANS is looked at, and even defined, as if limited to motor fibers and excluding sensory fibers, a more comprehensive definition is that the reflex arcs of the ANS comprises both a sensory (or afferent) arm, and a motor (or efferent, or effector) arm.

Sensory neurons

The sensory arm is made of “primary visceral sensory neurons” found in the peripheral nervous system (PNS) in “cranial sensory ganglia:" the geniculate, petrosal, and nodose ganglia, appended respectively to cranial nerves VII, IX, and X. These sensory neurons monitor the levels of carbon dioxide, oxygen, and sugar in the blood; arterial pressure; and the chemical composition of the stomach and gut content. They also convey the sense of taste, a conscious perception.

Blood oxygen and carbon dioxide are, in fact, directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion.

Primary sensory neurons project (synapse) onto “second order” or relay visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), which integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, which detects toxins in the blood, and the cerebrospinal fluid. It is essential for chemically induced vomiting and conditional taste aversion (the memory that ensures that an animal that has been poisoned by a food never touches it again).

All this visceral sensory information constantly, and unconsciously, modulates the activity of the motor neurons of the ANS.

Motor neurons

Motor neurons of the ANS are also located in ganglia of the PNS, called “autonomic ganglia.” They belong to three categories with different effects on their target organs: Sympathetic, parasympathetic, and enteric.

Sympathetic ganglia are located in two sympathetic chains close to the spinal cord: The prevertebral and pre-aortic chains. Parasympathetic ganglia, in contrast, are located in close proximity to the target organ: The submandibular ganglion close to salivary glands, paracardiac ganglia close to the heart, and so forth. Enteric ganglia, which, as the name implies, innervate the digestive tube, are located inside its walls and collectively contain as many neurons as the entire spinal cord, including local sensory neurons, motor neurons, and interneurons. It is the only truly autonomous part of the ANS and the digestive tube can function surprisingly well even in isolation. For that reason, the enteric nervous system has been called “the second brain.”

The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” (also called, improperly but classically, "visceral motoneurons") located in the central nervous system. Preganglionc sympathetic neurons are in the spinal cord, at thoraco-lumbar levels. Preganglionic, parasympathetic neurons are in the medulla oblongata (forming visceral motor nuclei: The dorsal motor nucleus of the vagus nerve (dmnX), the nucleus ambiguus, and salivatory nuclei) and in the sacral spinal cord. Enteric neurons are also modulated by input from the CNS, from preganglionic neurons located, like parasympathetic ones, in the medulla oblongata (in the dmnX).

The feedback from the sensory to the motor arm of visceral reflex pathways is provided by direct or indirect connections between the nucleus of the solitary tract and visceral motoneurons.

Function

Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest."

However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually opposing fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:

Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.

The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.

During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.

The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.

Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.

Neurotransmitters and pharmacology

At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine) to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla:

At sweat glands, the neurotransmitter is acetylcholine, which acts on muscarinic receptors.

At the adrenal cortex, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors.

Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which will act on adrenoceptors, producing a widespread increase in sympathetic activity.

In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter, to stimulate muscarinic receptors.

The following table reviews the actions of these neurotransmitters as a function of their receptors.

Gershon, M. D. 1998. The Second Brain: The Scientific Basis of Gut Instinct and a Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine. New York, NY: HarperCollins Publishers. ISBN 0060182520

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